A one-package polyurethane sealant composition comprising an isocyanato-terminated isocyanate prepolymer containing a polyester polyol or polyether polyol component cures as it forms a urea bond chiefly on reaction with the water (moisture) present in the atmosphere. Just because it cures in the presence of moisture, such a one-package sealant composition suffers a considerable reduction in the reaction rate under low-temperature, low-humidity conditions.
To overcome this disadvantage, one-package heat-curable urethane adhesives and paints which cure on thermal dissociation have heretofore been made available by way of blocking the isocyanate or the polyol component.
Thermally curable polyurethane adhesives and paints based on temperature-sensitive amine or polyol curing agents as well as hot-melt type one-package polyurethane sealants have also been proposed.
However, such polyurethane sealant compositions are low in the rate of curing reaction particularly in the low-temperature, low-humidity environment so that they tend to sag (flow) or be deformed by vibrations or the like. In the case of a heat-curing sealant, heating at a high temperature is required.